438 



MARINE INVERTEBRATES 



JP. hfistatiis. There are several very beautiful pectens on the Pacific 

 coast of the United States, and two of these one of northern and the other 

 of southern range resemble very closely the two Atlantic species of 

 relatively north and south range, P. islandicus and P. irradians or dislo- 

 catus. The first is P. hastatus of Puget Sound. Valves of this exquisite 

 shell are even occasionally found in California. It has very much the 



aspect of P. islandicus 

 unequal ears and an ovate 

 form. The most curious 

 thing about this shell is the 

 fact that a different system 

 of sculpturing exists upon 

 the two valves. Upon one 

 the ribs are even and the 

 whole surface is covered 

 with raised scales ; upon 

 the other the ribs are alter- 

 nately of large and small 

 size. Color rose-pink, with 

 deeper bands; one valve 

 lighter-colored, suffused 

 with delicate pink. Height 

 two and a half inches. 

 (Plate LXXIX.) 



JP. cequisulcatus. The 

 Southern species above re- 

 ferred to as resembling P. 

 irradians or dislocatus. It 

 has about twenty stout, 

 well-rounded ribs, with concentric wrinkles between them. The " ears " 

 are equal. The size and shape are as in P. irradians. The coloring is 

 whitish, varying to shades of brown, and excentrically marked with 

 spots or waving transverse lines of chestnut or horn. 



Pectfn irradians, tj'pical form. 



ORDER EULAMELLIBRANCHIATA 



With the pectens we leave behind all pelecypods with the 

 true filamentous type of gill. In the order Eulamellibra'Hclii- 

 ata we find all the families to be possessed of the basket- 

 work or crisscross branchial structure. Each gill is fun- 

 damentally formed of filaments which project downward, and 

 then suddenly turn upon themselves and ascend ; the ascending 

 and descending limbs .are connected, however, by processes of 

 presumably vascular function, and adjacent filaments are con- 

 nected by truly vascular channels. The closer connection of the 

 filaments to form of each gill a sort of plate is further effected 



